The invention relates to a magnetic resonance experiment in which a sample under test is subjected to a sequence of gradient magnetic fields and radio frequency pulses and resultant pulse echoes are monitored. The invention also relates to apparatus for performing such an experiment.
In a typical magnetic resonance imaging experiment, after initial excitation of selected spins, in which spins at different spatial positions are encoded with different resonant frequencies by the superposition of a linear magnetic field gradient upon the main field, the signal detection must take place in the presence of a similar field gradient, in order for the spatial encoding to be maintained. In the presence of this gradient, spins have different precessional frequencies, and so dephase. Fourier imaging techniques rely on phase-encoding, that is the application of a known gradient (in the second imaging direction), between initial excitation and signal acquisition. The value of this phase-encoding gradient is incremented from scan to scan. Thus signal acquisition may not occur immediately after excitation. In order to be acquiring at the moment when the spins are in phase, it is necessary to first apply a negative gradient (in the first imaging direction). This allows spins to dephase by a certain amount before signal acquisition commences. During acquisition, as the spins rephase, the signal intensity rises to a maximum and then decreases again as the spins continue to precess at different frequencies. This signal is called the echo. The fourier transform of this echo yields the frequency (and hence spatial) profile of the excitation.
When performing large flip angle gradient recalled echo experiments with slice selective pulses, a build up of transverse coherences occurs, causing transverse steady state artefacts in the resultant image, due to the formation of undesired spin echoes and stimulated echoes. The required T.sub.1 -dependence of the signal intensity is essentially destroyed by these extra signals.
Methods to "spoil", or destroy, these transverse coherences have employed variable addition gradient pulses or radio frequency methods in which successive pulses have little or no phase coherence. The former suffers not only because "extra" gradients can in fact refocus undesired coherences, but also as they introduce further eddy currents, which result in localised distortions in the magnetic field. Radio frequency methods in which phase coherence between successive pulses is removed are difficult to implement precisely and some transverse coherences persist and, in fact additional gradients are also required to prevent cancellation between the phase-encoding required in image formation and the phase offset supplied to remove coherence between pulses.
In accordance with one aspect of the present invention, a magnetic resonance experiment in which a sample under test is subjected to a sequence of gradient magnetic fields and radio frequency pulses and resultant pulse echoes are monitored is characterised in that at least one of the radio frequency pulses is a prefocused, phase-modulated, time-asymmetric radio frequency pulse.
In accordance with a second method of the present invention, apparatus for performing a magnetic resonance experiment comprises magnetic field generating means for subjecting a sample under test to a sequence of gradient magnetic fields; an RF pulse generator; and means for monitoring resultant pulse echoes and is characterised in that the RF pulse generator generates a prefocused, phase-modulated, time asymmetric RF pulse.